Since line-in counts as a recording source there's no straightforward way of configuring it as a playback source using the gnome gui. I promise, this won't be another rant about gnome going backwards in usability.. but it is. Instead let's just fix the issue. What you need to do is enable pulseaudio loopback so that the audio from the recording source loops back as a playback source. simply execute "pactl load-module module-loopback". If you want it to stick around after reboots do "sudo sh -c 'echo "load-module module-loopback" >> /etc/pulse/default.pa'" I ran into this problem as I'm moving away from traditional workstations. Instead my "workstation" (singular) is in fact multiple devices these days. Each doing their own bit of processing and user input, depending on their strengths. This has forced me to route audio through bluetooth and cords like I'm some half-assed dj. It's outright depressing to me that we don't have simple gui options for this in linux. It's 2025. Still, it's likely it's just gnome again, this option very might well be there in kde (as it usually is). But I'm hanging in here, I'll give gnome a shot until 25.10 is out.